THE SEA OTTER IN THE EASTERN PACIFIC OCEAN 193 



immigration) at a rate of about 10 to 12 percent per year. In a 

 region that includes both densely populated areas (where winter- 

 spring mortality occurs) and adjacent unpopulated habitat, the 

 rate of growth may be about 4 to 5 percent per year. 



Optimum population density 



No study has yet been undertaken to indicate the productivity 

 of the sea otter habitat or to show what critical differences in food 

 productivity exist in different habitats. Because such studies are 

 not available it is not possible to compute, on the basis of food 

 resources available, the number of sea otters that any particular 

 habitat may support. 



On the basis, however, of the observed behavior of several island 

 populations of sea otters it is possible to reach tentative con- 

 clusions as to the approximate optimum population density and 

 which areas are overpopulated or underpopulated. 



At five locations (Rat, Amchitka, Tanaga, and Kanaga Islands, 

 and the Delarof group) we observed large otter populations which 

 subsequently became greatly reduced (table 37 and fig. 78). The 

 figures available do not necessarily represent absolute maximum 

 and minimum populations because of the infrequency of observa- 

 tions. From these data it appears, however, that a growing sea 

 otter population in an unexploited habitat may reach a temporary 

 maximum density of 40 or more animals per square mile of habitat. 

 After such a high density is attained, a period of population re- 

 duction occurs, either through emigration, a high rate of mortality, 

 or both concurrently during seasons of stress. That a depleted 

 feeding habitat may support about 10 to 15 otters per square mile 

 is indicated by the observation that the five densely populated 

 areas dropped to that level. 



Table 37. — Changes in sea otter population density in six areas 



High population Low population 



Interval Means of 



Island Date Number i Date Number ^ years reduction ^ 



Rat 1943 31 1949 10 6 Mortality (?). 



Amchitka- 1943 41 1949 13 6 Mortality. 



Delarof 1959 21 1965 10 6 Mortality (?). 



Tanaga 1953 (3) 1959 14 6 Emigration. 



Kanaga 1959 26 1962 12 3 Emigration. 



Adak- 1962 40 1965 24 3 Emigration and 



mortality. 



Mean 30 12 



'l 1 Otters per square mile of feeding habitat (waters 30 fathoms or less in depth). 



2 It ig known that emigration took place because rapid population increase in nearby areas 

 was observed. Mortality was observed and documented only at Amchitka. At Rat Island and in 

 the Delarofs a condition similar to that at Amchitka would be expected. 



2 Unknown but presumed to be high on the basis of reports that otters there numbered in the 

 thousands (Hooper, in Jones, 1953; Lensink, 1960). 



